Thick film hybrid circuits having high packing densities are well known in the art. Hybrid circuits are multilevel interconnected circuits consisting of a semiconductor or integrated circuit chip mounted on a ceramic insulator substrate. The circuit may be designed with termination pads for attaching semiconductor chips, connector leads, capacitors, resistors, and the like.
Thick film electrical components are made from inks or pastes formulated with organic solvents, binders and materials which will make the ink or paste either a conductor, a resistor, or a dielectric. The ink is screen printed in a selected pattern onto a nonconductive substrate, such as a ceramic or glass. The printed substrate is then dried to remove the organic solvent constituents contained in the paste and then the substrate is fired to sinter or fuse the inorganic constituents to bond the printed film to the substrate.
Precious metal conductor pastes are prepared from a noble metal, or combination of noble metals, such as gold, silver, platinum, palladium, iridium, rhodium, ruthenium, and osmium. Resistor paste materials are prepared from a variety of substances such as carbon, thallium, indium, ruthenium, and the like. Dielectric paste materials are prepared from materials such as glasses, ceramics, glazes and ferro-electric materials. Precious metal conductor, resistor, and dielectric materials are designed to be compatible with each other.
In a typical thick film firing process, the printed and dried ceramic substrate is passed through a multi-zone belt furnace. The speed of the belt is constant and the substrate progresses through heating zones of increasing temperature, typically up to about 700.degree.-1000.degree. C. The substrate is maintained at peak temperature for a predetermined period of time, generally about 5-15 minutes, after which it is cooled to ambient temperature. The zones in the furnace are commonly referred to as the burnout zone (with temperatures up to about 600.degree. C.), the hot zone (the peak temperature) and the cooling zone (ambient temperature). The presence of an oxidizing atmosphere (air) during the firing process serves to promote the sintering and bonding processes which occur between the thick film and the substrate. The presence of an oxidizing atmosphere also serves to oxidize and remove the carbon-based vehicles which are present in the paste for the purpose of improving the rheological properties of the paste during the printing step. The organic vehicles are oxidized to volatile by-products such as carbon monoxide, carbon dioxide, and water vapor which are readily swept from the furnace by the exiting air atmosphere.
Hybrid circuits are generally built in three dimensional structures by employing multiple layers of electrical components. Dielectric insulating layers are placed between conductor layers to permit the conductors to cross over one another. The insulated overlapping conductor layers are interconnected through vias formed through the insulating layers and filled with conductor material. The vias are holes formed in individual ceramic insulator layers which are filled with conductor metal paste. After being sintered, the vias become dense conductor metal interconnections.
Base metals, such as copper, have been substituted for precious metals in the thick film conductor pastes because of their lower cost and better physical properties (solderability and conductivity). Because copper will be oxidized in an air atmosphere during a firing step, an inert or neutral atmosphere, such as nitrogen, must be employed during firing. The inert atmosphere, however, in addition to not oxidizing copper components, also will not oxidize and remove the carbon-based vehicles from the paste in an efficient manner. In the absence of an oxidizing agent in the firing atmosphere, a portion of the organic components may pyrolyze during the firing process.
Low levels of oxygen or other oxidizing agents have generally been added to the inert firing atmosphere to oxidize and remove the organic paste vehicles. In a typical copper compatible thick film process, the printed ceramic substrate is fired in a belt furnace under an atmosphere which is substantially inert (nitrogen) but which is doped with small amounts of an oxidizing agent (typically up to about 100 ppm oxygen) to react with the organic vehicle. The burnout zone in the belt furnace is generally where the organic vehicles are burned out and removed, i.e., the zone before the furnace reaches peak temperature. The addition of the oxidizing agent is restricted to the low temperature burnout zone. There must be good separation between zones since an appreciable quantity of oxygen leaking into zones having temperatures above about 600.degree. C. will cause oxidation of the copper.
The use of an oxidizing dopant in an inert atmosphere during the thick film firing process has several disadvantages. The concentration of dopant in the inert firing atmosphere must be precisely controlled, and at such low concentration levels, such control is difficult. The oxidizing agent tends also to oxidize the copper conductor creating an adverse change in the electrical characteristics and soldering properties of the conductor because of the formation of copper oxide coatings. In addition, the carbon-based vehicles generally do not react efficiently with small amounts of oxygen present in the firing atmosphere and tend to form free carbon (soot) which can deposit on the walls and on the heating elements of the furnace, and on the substrate itself. Such deposits must periodically be removed adding to the cost of the operation. Carbon deposits on the substrate create an unacceptable appearance and can cause a defective circuit especially if the carbon deposit is between the layers of a multi-layer hybrid structure.
The use of conventional oxidizing dopants in an inert atmosphere during the thick film firing process has other significant disadvantages. Each conductor, resistor and dielectric component in the circuit usually has a different chemical composition and hence has a different chemical processing requirement in the atmosphere during the firing process. Each electrical component must usually be printed and fired separately adding additional cost to the procedure.
U.S. Pat. No. 4,234,367, issued to Herron et al. and assigned to International Business Machines Corporation, discloses a firing process for thick film circuitry of copper based conductors in an ambient of hydrogen and water vapor in a specified defined ratio followed by a sweep with an inert gas. The atmosphere is said to be reducing to copper but oxidizing to carbon.
U.S. Pat. No. 4,622,240, issued to Yext et al. and assigned to Air Products and Chemicals, Inc., discloses a firing process for copper based conductor thick film electrical components which involves using nitrous oxide as an oxidant in an inert atmosphere containing incidental amounts of water vapor.
Palanisamy et al. compared the thermodynamics for oxidation of copper and carbon based on the premise that pyrolysis or incomplete removal of the organic binders in the paste results in formation of carbon which must be removed by oxidation, Proceedings of the 1986 International Symposium on Microelectronics (ISHM), pp. 848-858. Palanisamy et al. considered the thermodynamics of mixtures of carbon monoxide-carbon dioxide and hydrogen-water vapor as reactive mixtures and concluded that the hydrogen-water vapor mixture was the superior dopant.
Thus, a number of methods are known to oxidize and remove carbon-based vehicles during the firing step in copper compatible thick film circuit forming processes. However, none of the methods is entirely satisfactory because each method requires very precise control of the concentration of the oxidizing dopant which is very difficult at such low levels. In addition, none of these methods provides a single firing atmosphere under which different electrical components, such as conductors, resistors, and dielectrics, can be fired at the same time. Accordingly, there is a need for an improved firing method in thick film forming processes which would efficiently oxidize and remove carbon-containing residues while maintaining reducing to neutral conditions for copper components. There is also a need for an improved process whereby different copper compatible thick film materials could be fired in a single atmosphere. Such a process could reduce the sensitivity of the conductor, resistor and dielectric materials to oxidants, improve the yield of the thick film forming process and reduce the cost of hybrid circuit manufacturing. The present invention provides such an improved process which allows the firing of different thick film components to be carried out in a single furnace atmosphere.